The goals of this research are to purify the glucocorticoid receptor from the kidneys of rats or larger animals and to analyze the receptor structure in terms of its two recognition functions: of specific steroids and specific nuclear sites. Cytosol receptors from adrenalectomized rats labeled with 3H-triamcinolone acetonide are purified and characterized by techniques including centrifugation and isoelectric focusing in glycerol gradients, and chromatography on DEA-cellulose, Agarose A-1.5m, 100-200 mesh, and Sephadex LH-20, G-50 and G-100. The conditions required for purification of the intact receptor (the holo-receptor) include the removal or inhibition of endogenous proteases in the kidney extracts. The most effective inhibitor is leupeptin, a bacterial tripeptide. A search for irreversible, more widely available inhibitors is in progress. During prolonged fractionations of cytosol in the absence of protease inhibitors, the receptors is cleaved into the smallest fragment containing the steroid-binding site (the mero-receptor). This process is facilitated by diluting, freezing and/or warming the cytosol. A study of potential activators of the protease(s) is in progress, including sulfhydryl groups and calcium ions. Analogous conversion of 10-11 S receptors to 2-3 S mero-receptors has been observed for other steroids and target organs, including human breast tumor estrogen receptors.